Interferometric comparator



oct; s, 194s..

G. E. MERRl-rr INTERFEROMETRIC COMP/muon v s sheets-sheet 1 vFiled Aug. 16. 1944 Ubbwss :v

Oct. 5, 1948. G. E. -MERRn-r 2,450,839

ITERFEROMETRI() COMPARAI'OR Filed' Aug. 16,-1944 e sheets-sheet 4 ja/u. 0

INVENTOR.

Oct. 5, 1948. G. E. MERRITT INTERFEROMETRIC COMPARA-TOR 6 SheeLbs-Sheet 5 Filed Aug. 16, 1944 vnwLfmr'oxf 'Byeorye E. menu/tb ATTORNEY Oct. 5, 1948. G. E. MERlTT 2,450,839 l INTERFEROMETRIC COMPARATOR Filed Aug. 1e, 1944 6 Sheets-Sheet '6 74 --11IIIIIHHO @,72 l

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INVENTOR. Y*

. UNITED STATES 2.450.839 I NTERFEBOMETBIC COMPARATOB George E. Merritt, Elmira, N. Y., assignor to'Bendix AviationCorporation, South Bend, Ind., a

corporation oi Delaware "P'F- NT "oFFlcs- The present invention .apparatus for comparing linear dimensions of parts by observation of y interference `fringes formed by reflections of portions oi a light beam from two nearly' parallel plane surfaces.l

--sppue'suonnumt 1e, 1944, serial No. 549,709 'r1 clans. (ci. :is- 143) relates to an interi'er-l Iometric comparator and more particularly to an 2 with such a device a constant temperature cabi- "net therefor which provides an accurately con trolled atmosphere free from suspended matter Interferometric gauges'have heretofore been I been limited -by' their structure to lthe measure-- while permitting access tothe device for adjustment and for thefintroduction and removal of the parts to be sauged.

Further objects and advantages will be appar- 1 v ent from thev following description taken in con- `-nection with the accompanying drawing. in

which:

Fig. 1 isa perspective view of a preierredembodiment oi' the invention as set up vin its air in which the optical ilats which" produce the fringes are readily accessible for cleaning but are at all times protected from contact with the work or any extraneous article.

It is a further object to provide such a devicev in which the dimension to be measured may vary quite widely from the separation .of the optical flats.

It is another object to provide vsuch a device A in which the various optical elements are readily azustable and the adjustment is accurately maintained.

' It is another object to provide such a device in which one of the optical iats is mounted with provision for coarse adjustment to' approximate desired location, and a fine adjustment for final positioning and securing the vdesired character f of interference fringes.

It is another object to provide such a vdevice in which the angle between the direct light beam and the light reflected from the optical iiats is very small, whereby the resultant interference conditioned cabinet:

Fig. 2 is a semi-diagrammatic view in perspecv tive of the air-conditioning system for the cabinet;

Fig. 3k is a perspective view of the comparator and viewing instrument removed from its cabinet;

Fig. 4 is a diagrammatic representation of the optical system of the comparator;

Fig, 5 is an enlarged detail partly broken away and in section showing the light source and the condensing and viewing lens systems;

Fig. 6 is an enlarged detail in perspective showing an adjustable mount for the prism;

Fis. '7 is an enlarged detail in perspective of the cap and light source holder; l

im. s is a. top plan view of the work holder I and mount for the optical ilats;

Fig. 9 is a front elevation thereof partly broken away to show theadjusting means, and l Fig. 10 is an end view of the same.

In Fig. 1 of the drawing there is illustrated a cabinet comprising a heavy rigid heat insulated frame l providing a chamber 2 t6 be air-conditioned. 'I'he chamber has a door 3 with a grilled air outlet 4, a back wall 5 having air inlet 6, a iront window 1 of suitable material such as plate glass and a front panel 8 on which the various controls and instruments are mounted and which has provision for access'to the interior of the chamber while preventing air leakage. v

The work holding part 8 of the interferometric in such position that the apex of they cone 33 tical flats substantially in the planell. Means a duct i3 leading downward from the air outlet opening4 in the floor 3 of the chamber, which duct co tains a sensitive thermostat I4, an electrical heating grid I5, and a cooling coil I0. A blower I1 operated by a motor I3 vdraws the air from the duct i3 and forces the air through a cleaning system comprising a filter chamber I and an electro-static precipitator |00, the discharge from which enters the upper portion of chamber 2 through the opening 0.

The temperature control is regulated by means of a manual adjustment I0| mounted on the front panel 0 whereby the thermostat I4 may control energization of the heating grid I3 or admission of cold water toA thecooling coil i3 as required to maintain the desired temperature. Inasmuch as the temperature control unitsland the air cleaning devices do not in themselves constitute a. part of the present invention further description thereof is deemed unnecessary.

An inlet |02 is provided for admitting outside air into the system. and the operation of the blower I1 is so controlled as to maintain a slight positive air-pressure in the chamber 2 so that any leakage would be in an outward direction thus preventing contamination of the air within the chamber.

In order to permit -manipulation of the gauge while preventing escape of air from the cabinet. the panel 0 is provided with an opening |03 v which is closed by a suitable ilexible septum |04 having arm holes |05, |06 therein. which arm holes are closed by sleeves |01, |08 with elastic self puckering wrist bands |00, ||0- adapted to llt closely around the wrists of the operator. In order to permit the entry and removal of articles from the chamber, the septum |04 is slit as shown at III. the slit being closed by a slide fastener H2.

The salient features of the optical system involved in the present invention are illustrated diagrammatically in Fig. 4. As there shown a source 20 of monochromatic light such as a tube/ containing helium excited to luminescence by passage of electricity therethrough is mounted closely adjacent a diaphragm 2| having an aperture 22 of. small diameter. The cone 23 of light traversing this aperture from,y the light source is directed upon the convergent lens 24 which is located at its focal distance from the light source whereby the cone of light is converted into a' beam 20 of parallel rays.

A pair of optical flats 20 and 21 are arranged in the path of the parallel beam 23 with their adjacent surfaces 20 and 20 substantially parallel and almost but not quite normal to said beam whereby light is reflected back from said surfaces 23, 20 in a beam 30, the axis 3| ot which makes a very small angle (A) with the axis 32 of the original beam. The lens 24 is preferably designed to have a long focal length Lin order to make the angle (A) small. In practice a focal length of 15 incheshas been found to be satisfactory. The reflected beam is converged by the lens 24 into a cone 33, the upper end of which passes through a small opening 34 in a dia-V phragm 33 closely adjacent the opening 22 in diaphragm 2l.

-A prism 33 is located just above the opening 34 strikes the prism close to its lower edge. JThe light so reected emerges laterally through a lens 31 which is arranged to superimpose the reilections from the two surfaces 23, 23 of the opfor observing the pattern so formed is provided s a suitable source 48 of high voltage such as a step-up transformer. As here shown thetransformer is energized from the power line 4l through a variable resistance unit 50 under the control of a manual switch 5 I.

The cap 43, as best shown in Fig. 7, is provided with a groove 52 for receiving the light source. having enlargements 33 and 04 formed to receive and cooperate with clips 53 and II which retain the light source in position but are readily exed to permit removal thereof. The bottom of the cap 43 is closed in order to form the diaphragm 2| which is light-tight except for the opening 22 formed directly below the light source 20 so as to permit the iight therefrom to be projected downwardly within the tube 42.

The observing lens system (Fig. 5) is mounted on a bracket 51 fixed on the upper endof the tube 42 and having acylindrical bearing 50 within which a lens tube 53 is slidably and rotatably mounted. The prism 30 is mounted in a holder 30 ilxed on the inner end of ,the tube 30, flexible ,shown in Fig. 7 which permits the prism 33 to be moved into close proximity to the light opening 22. Preferably this recess is arranged to permit the prism to be advanced until its lower edge 33 I may partially intercept the cone of light emerging from saidrlight opening so as to allow the angle (A) between the direct and the reflected beam to be made very small. Thus in one preferred embodiment of the inventionthe openings 22 and 34 are each one-sixteenth of an inch in diameter, and the focal length of the lens 24 is sixteen inches. When the instrument is arranged to make the openings 22 and 34 tangent to each other, this makes the angle A have a value of about three minutes thirty-eight seconds. Adjustment of the tube Il is locked by suitable means such as a thumb screw 34.

A telescopio tube 03 is slidably mounted on the outside of the cylindrical bearing 3l and carries in its outer end the eye piece comprising the viewing lenses 30. 40. Adjustment of the tube Il for focusing purposes is permitted by sliding the tube on the bearing 33. the adjustment being locked by the clamping means 33.

As shown in Fig. 3. the work holder and mounting means 3 for the optical ilats rests on the floor 3 of the cabinet 2 directly below the optical system I0 but entirely disconnected therefrom. This element comprises a base 0I on which is lilxedly mounted the lower optical flat 21 in a 4 rigidly attached the upper ends of flexible hinge members 12, 13.

The lower ends of the hinge members are xed to anchorages 1l, 16 which areJverticaliy adjustable by diiterentially threaded micrometric screws 16, 11. Coarse adjustment otthe hinge mounts casacca therewith is inserted in its place. If the diameter of this artice is not the same as that of the gauge block, the upper optical iiat 26 will be correspondingly tilted with respect to the lower is provided by means of threaded bushings 18, 5 flat so that the angle between the surfaces 28, 29

18 receiving the lower ends of the screws 16, 11. and held in .adjusted position by set screws 88. 8|, while ilne adjustment is secured by--subsequent rotationof the screws 16. 11.

`will'ibe changed. This change in angle will be signalized by a corresponding change in the number of fringes observed inthe eye-piece of the instrument, which vchange may form the basis for An abutment) forming one jaw ofthe -com- 1o an empirical calculation of the variation of the parator gauge As provided by a block 62 vfixed 'y on the i'ront of the base 68 with adjusting means 82 and clampingoneans 84. The upper surface Il pf the abutment 82 is ilat and parallel with the mounting ring 86 for the upper optical ilat 26.

The lower surface 8l of the abutment 86 is alsoy nat and substantially horizontal when the ring v6I is horizontal but may be tilted slightly by means of the ad-.iustment 88. A cylindrical hardened contact member 86 is preferably mounted in the jaw 86 radially to the ilat 28 and projecting slightly beyond the surface 81, so as to provide a ,point contact with a cylindricalwork piece inserted between the jaws of the gauge. Means for holding the Work 89 between the abutment jaws 82 and 88 'is provided in the form of an arm 98 article from standard dimension.

Although certain structure has been illustrated and described in detail, it Awill be understood that f other embodiments of the invention are possible and that various changes maybe made in the design and arrangement of Athe parts illustrated without departing from the spirit of the invention as defined inthe claims appended hereto.

What is claimed is:

o l. An interferometric gauge comprising a pair of relatively movable optical flats, means for determining the relative position of the ats in accordance with the critical dimension of the article to be gauged, means including said ilats forming an optical interference pattern which is controlled by the relative position of the iiats, means for viewing said pattern including a, telei f scope, a cabinet housing the gauge and providing a fixed mount therefor, universallyadjustable pivoted. at 9| to the base 88 and having a spring so means for mounting the telescopeon the extelweight of the ring is used to uit itin a direction to close the gauge jaws 82, 86.

The tripod 4| is provided with feet 93, 84 and 96 which are adjustable to vary the angle of incidence of thev light beam 25 on the optical flats.

' In this manner the reflected beam may be directed through the opening $4 onto theprism 66 as described.

Irior of the cabinet, flexible) sealing means for preventing leakage of air from the cabinet, and. air-conditioning means for the cabinet including Theiupper at 26 is preferably made slightly 45 prisesa circulatory system including temperature wedge-shapedso that the light reected from its upper surface 86 will be deected laterally suill- In operation, assuming that the work to be dilioned ail' abOVe and in the rear 0f the gauge.

gauged has remained in the cabinet a sumcient ltime to have .reached the stabilized temperature,

controlling means, circulating means, a mechanic. cal filter and an electro-static precipitator, the system being arranged to withdraw air from in diront of and beneath the gauge and supply conto' cause the conditioned air to ow over and around the gauge.

4. In an interferometric comparator a source of monochromatic light, avcondensing lens forminstrument, and in particular said abutmcnts, are ing a beam oi.' parallel rays from said light source,

then adjusted to secure a suitable interference pattern as observed through the telescopiceyepiece 39. 482 This interference pattern is formed by the diilerence in phase of the light waves reilats and superimposed in the field of view of the eye-piece by the projection lens 31,- If the surfaces 28, 29 are exactly parallel there will be no interference fringes. but deviations from such parallelism cause alternate reinforcements andv d5 stantially parallel surfacesforms an interferencel cancellations of the reflected light waves in accordance with their phase relation. An aincrease inthe angle between the two ilatsurfaces 28,'28

e accordingly increases the number of fringes. in

i the ileld of view.

a light transmitting optical ilat which is slightly i wedge-shaped in cross-section, arranged in the path of said beam with its emergent surface substantially normal' thereto, a. second optical ilat `flected from the two surfaces 28, 29 of the optical 5located in the Path of said beam spaced in 4subpattern a tilting mount for one of said flats including a pair of exible plates rigidly aixed to the ats in alignment to form a spring hinge, gauge jaw members rigidly connected to theats laterally with respect to the hinge, whereby introduction of an element to be gauged between said jaw members causes relative angular dislacement of the ats. the axis of said hinge ing slightly oiIset from 'the center line of the the article 88 which it is desired t0 emrlpare 75'ats in a direction away from the gauge Jaws,

optical means for observing the interference pattern caused by said displacement, and means for mounting said observing means with provisions closely adjacent the light source whereby said condensing lens "converges the reflected beams of light upon the prism near said edge.

'1. An interferometric comparator as set forth in claim 4 in which one of said jaw members is mounted for angular adjustment about an axis parallel to said hinge axis whereby the surfaces of the jaw members may be brought into parallelism. y g f 8. In combination, a cabinet including a chamber having a flat floor, means for maintainingthe chamber at a constant temperature. an interferometric comparator comprising a light source. optical projecting and viewing systems, a tripod frame supporting said light source and optical systems on the exterior of the cabinet, a pair of optical flats, means for supporting the flats inde` pendently of said tripod frame in substantially parallel relation on the floor within the chamber in the beam of light projected from the light source. means for adjusting the tripod frame with respect to the cabinet to thereby adjust the angle of incidence of said beam on the optical flats. and flexible closure means attached to the tripod frame and the wall of the cabinet for preventing the escape oi' air from the chamber without interfering with the adjustment of the tripod frame; 9. An apparatus as set forth in claim 8 in which the viewing system includes a telescope, said apparatus also comprising means for removing suspended matter from the air in the cabinet and 45 8 optical nat, a movable the movable fiat in substantially parallel relation to the stationary fiat including a.` pair of aligned flexible hinge plates fixed to one of the flats and adjustably anchored to the other fiat. a 89.1186 l jaw fixed to the stationary fiat having a plane surface parallel to the surface of the stationary flat, and a movable' gauge jaw attached to the. movablesfiat having a'cylindrical surface, the axis of which is parallel to the surface of the movable flat and normal to theiplane of the hinge plates.

11. An interferometric gauge as set forth in claim 10 in which said fixed gauge jaw has an abutment surface parallel to the axis of the hinse and substantially perpendicular to the plane surface of the jaw to position a part to be measured f/ in `the Jaws, and ay spring actuated member arranged to hold said part in engagement wit Rthe abutment surface with a substantially uni,

form pressure. i i GEORGE E. MERnrrr. l

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date f 1,901,632 Chamberlain Mar. 14, 1933 2,286,621 Hurley June 16, 1942 2,312,888u Everest Mar. 2,' 1943 I FOREIGN PATENTS Number Country Date 239,120 Germany Oct. 10, 1911 OTHER REFERENCES A New Interferential Dilatometer.Y by I. G. Priest; Bureau of Standards Scientic Paper No.

.40 865; pub. 1920; pp. 669 to 678 and plate facing page 674 cited. (Copy in Div. 7, U. S. Pat. Off.)

Interference Methods for standardizing and Testing Precision Gage Blocks." by C. G. Peters et al.; Bureau of Standards Scientific Paper No. 436; pub. 1922; p. 685 cited. (Copy in Div. '1. U. anton.)

fiat, means for supportinl 

